Varistors, i.e., voltage-dependent resistors, display variable impedance, depending on the current flowing through the device or the voltage across it. The properties of varistor materials make them advantageous for uses such as electrical stress control devices and surge arrester, i.e., surge protection, devices.
Electrical stress control devices are used to contain and manage electrical stress, e.g., when shielded power cables are terminated or connected. When the insulation shield is removed from a cable, the electrical field is concentrated at the cutback point, causing high electrical stress. Electrical stress control is needed for such application of terminations or connecting cables.
Surge protection devices protect against surges generated by electromagnetic effects, such as lightning or electrostatic discharge caused by a variety of effects. As such, surge protection may be applied at the mains input to combat disturbances on the mains supply external to the operating equipment or internally generated overvoltages. A surge protector may either attenuate a transient by filtering or divert the transient to prevent damage to the load.